Automatic frequency control



, Feb. .25, '1941. R, D 2,233,198

AUTOMATI C FREQUENCY CONTROL Filed June 30, 1938 Robert B Dome,

His Attorney.

Patented Feb. 25, 1941 AUTOMATIC FREQUENCY CONTROL Robert B. Dome, Bridgeport, Conn, assignor to General Electric Company, a corporation of New York Application June 30, 1938, Serial No. 216,716

4 Claims.

My invention relates to an automatic frequency control arrangement for signalling apparatus and is an improvement on the automatic frequency control means disclosed in an application of Stanford Goldman, Serial No. 216,715, filed June 30, 1938 and assigned to the same assignee as the present application.

The signaling receiving apparatus constructed in accordance with the arrangement of said Goldman application operates to correct both for improper tuning of the signalling apparatus to a selected signal frequency and to correct for any slight frequency drift of the local oscillator of such apparatus. In operation, the automatic frequency control means maintains substantially constant the frequency of intermediate frequency oscillations supplied to an intermediate frequency amplifier. The arrangement of Stanford Goldman has the limitation that its range of frequency correction is relatively small and, under certain conditions of operation, a certain amount of frequency modulation of the local oscillator at an audio frequency rate may, if not prevented by the provision of rather expensive filter networks, lead to undesirable operating conditions.

An object of my invention is to provide an improved and simplified automatic frequency control for a signalling apparatus.

A further object of my invention is to provide 30, a reliable and relatively inexpensive arrangement for automatically controlling the frequency of a local generator of oscillations and one which does not frequency modulate the local oscillator at an audio frequency rate.

Another object of my invention is to provide a control circuit for automatically controlling the frequency of a generator of local oscillations by an arrangement having a relatively wide range of frequency correction yet possessing the characteristics of sturdiness and reliability. Those desirable characteristics result from the use of a resistor having a non-linear resistance characteristic in a control circuit which utilizes the change in resistance to effect a consequent change in the frequency of the generator of local oscillations.

A further object of my invention is to provide an amplifier arrangement in which one of the electron discharge signal amplifiers operates simultaneously as a signal amplifier and as an amplifier of a direct current control potential and one which, therefore, requires relatively few additional component parts other than those already present in the receiver.

, The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be underv stood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates an embodiment of my invention and Fig. 2 illustrates a modification thereof.

Referring to Fig. 1 of the drawing, I have illustrated my invention as embodied in a superheterodyne type of radio receiver in which an antenna ground system H], H is connected to supply signal oscillations to a signalling apparatus, represented conventionally by the rectangle l2, which may comprise one or more stages of radio frequency amplification, a mixer or frequency changer for changing the oscillations of signal frequency to oscillations of a lower or intermediate frequency, and one or more stages of intermediate frequency amplification for amplifying the oscillations of intermediate frequency. The mixer, or frequency changer, may be of the converter type in which certain of the tube elements are connected in a manner to constitute elements of a local oscillator and the mixing of signal and locally generated oscillations is accomplished electronically, or the mixer stage may be of a more simplified type wherein the signal oscillations and oscillations locally generated by a separate electron discharge device suitably connected to oscillatory circuits, are supplied to the same or different grids of an electron discharge mixer tube. My invention is illustrated with a mixer stage of the latter type in which the mixer tube, not shown, has a control grid connected to receive signal oscillations and a second grid, preferably screened from the signal grid, connected through a condenser 13 to receive oscillations from a local oscillator 2|.

The local oscillator 2| is here shown as comprised by an electron discharge device 9 provided with a grid [5, a cathode l6 and an anode H, the grid I5 and the cathode [6 being connected across a parallel tuned circuit constituted by the secondary winding I8 of the transformer l4 and the variable condenser l9, while the anode l! of the device 9 is connected through a primary winding 20 of the transformer 14 to the positive terminal of a source of space current, not shown. It will be understood that the negative terminal of the space current supply is connected through ground to the cathode [6 of the device 9. The oscillator condenser I9 is shown, as indicated by the broken line, mechanically connected for unicontrol operation with the input tuning condensers 22 provided for the purpose of tuning the radio frequency stages and the mixer stage to the frequency of the signal oscillations. Included in the apparatus represented conventionally by the rectangle I2 is one or more stages of intermediate frequency amplification whose output is supplied to the primary 23 of an output intermediate frequency transformer 24. The transformer 24 is provided with a pair of windings 25, 26 which are tuned by the respective condensers 21, 23 to the intermediate frequency. One terminal of the transformer winding 25 is maintained at ground potential for alternating currents of the intermediate frequency by a condenser 29. The other terminal of the winding 25 is connected by a conductor 36 to a center tap on the transformer winding 26. The connection of the transformer windings 25, 26 in this manner provides, with the condensers 21 and 28, a frequency discriminating network which operates in a manner well understood in the art. A detailed explanation of the operation of the frequency discriminating network may be understood from the explained operation of a very similar network disclosed in the Proceedings of the Institute of Radio Engineers for March 1937, page 289.

Each terminal of the winding 26 of the transformer 24 is connected to an anode provided in the respective electron discharge diode rectifiers 3| and 32. The cathodes of the diode rectifiers 3|, 32 are connected together through a pair of resistors 33, 34, across which an automatic frequency control potential may be developed by a departure of the intermediate frequency oscillations from a normal value. An additional resistor 35 is included in the last named connection for a purpose to be explained hereinafter. The lower end of the winding 25 of the transformer 24 is connected through a filter comprised by the condensers 29 and 36 and a resistor 37 to the junction between the resistors 33 and 34. A capacitor 84 is connected from the cathode of diode 3| to ground for the purpose of grounding this cathode for audio frequency currents.

The diode rectifier 32 rectifi-es the modulated intermediate frequency oscillations and produces across the resistor 33 an audio frequency potential corresponding to the modulation component of the modulated signal oscillations. This audio frequency potential is supplied through a condenser 38 to a voltage divider 39 and a portion of the potential drop across the voltage divider is supplied to a control grid 40 and through ground to a cathode 4| provided in an electron discharge device 42. The control grid 40 of the device 42 is biased to a normal operating potential by a cathode biasing resistor 43 and shunting condenser 44. The device 42 has an anode 45 which is connected through a resistor 46 to the positive! terminal of a high voltage space current supply, not shown. The amplified audio frequency oscillations appearing in the anode circuit of the device 42 are coupled through a condenser 41 to a grid 48 provided in the electron discharge device 49. The discharge device 49 has a cathode 50 maintained atground potential for alternating currents by a condenser a suppressor grid 52 directly connected to the cathode 50, a screen grid 53 connected tothe positive terminal 54 of a high voltage space current supply, not shown, and an anode 55 which is connected through the primary 56 of an audio frequency output transformer 51 to the positive terminal 54 of a high voltage space current supply, not shown. The transformer 51 is provided with a. secondary 58 which is connected to a loud speaker 59 or other translating device.

For the purpose of automatically controlling the frequency of the local oscillator 2| to maintain the frequency of the intermediate frequency oscillations supplied to the transformer 24 substantially constant in value, an automatic frequency control circuit 60 is provided. This automatic frequency control circuit operates to control the frequency of the local oscillator 2| by virtue of a change in the reactance reflected by the magnetic coupling of a winding 6|, provided in the transformer l4, into the frequency determining oscillatory circuit l8, IQ of the local oscillator. The impedance of the circuit 60 is comprised by the reactance of the winding 6|, by the reactance of a by-pass condenser 62, and by the resistance at a given moment of a resistor 63. The resistor 63 preferably has a positive temperature coefllcient of resistance. This resistor may, for example, be the filament of a small two-volt incandescent lamp. The resistor 63 has a terminal resistance dependent upon the temperature at a given moment of the resistor which, in turn, is dependent upon the current flowing through the resistor at that moment. To supply this current, the resistor 63 and the transformer winding 6| are connected in series through a conductor 64 and a resistor 65 between the cathode 59 of the device 49 and ground. Since the negative terminal of the high voltage space current supply, not shown, for the device 49 is also grounded, this connection of the frequency control circuit 60 in the cathode lead of the device 49 places the frequency control circuit 66 in the output circuit of the device 49. The resistor 65, in addition to'providing a proper bias for the grid 48 of the device 49, aids the condensers 5| and 62 in filtering audio frequencies, appearing in the output circuit of the device 49, out of the control conductor 64 so that the current flowing through the resistor 63 is sub:- stantially a non-pulsating unidirectional current.

The automatic frequency control circuit is completed by connecting the upper terminal of the resistor 34 through a conductor 66 and a resistor 61 to the grid 48 of the device 49. A by-pass condenser 68 maintains the conductor 66 at ground potential for oscillations of audio and intermediate frequencies.

An automatic volume control for the receiver is provided by supplying the unidirectional potential appearing across the resistor 33 through a filter comprisedby the resistors 69, 69a and condensers ill, TI to an automatic volume control circuit 12 thereby to control in well known manner the gain of the one or more stages of amplification included in the receiving apparatus l2 to which this control circuit is con nected. v

In explanation of the operation of the automatic frequency control arrangement, let it be first assumed that the frequencyof the local heterodyning oscillations, determined by the oscillatory circuit l8, I9, is such that when mixed with the signal oscillations, an intermediate or beat frequency oscillation results, the frequency of which is exactly thejfrequency to which the circuits 25, 21 and 26, 28 are tuned. In such case, the average unidirectional potential appearing across the resistor 34 is substantially equal and opposite in polarity to that appearing across the resistor 33 and the bias on the grid, 48 of the discharge device 49 is a normal operating bias determined by the potential drop across the resistor 65 added to that across the resistor 63. Under this assumed condition of operation, the frequency control circuit 60 refiects into the oscillatory circuit l8, IQ of the local oscillator 2| a certain reactance, determined by the steady-state space current flowing in the device 49 from the cathode 50 to ground, which is just sufficient to insure that the output frequency of the local oscillator 2| differs from the frequency of the signal oscillations by a frequency equal to that to which the windings of the transformer 24 are tuned.

Assume as a second condition of operation that a departure or drift is experienced in the output frequency of the local oscillator. The intermediate frequency no longer has the optimum or normal frequency which it previously had but will depart up or down in the frequency spectrum from its former value depending upon the direction of the frequency drift of the local oscillator, As explained in the Proceedings of the Institute of Radio Engineers paper previously referred to, the frequency discriminating network 25, 21 and 26, 23 now supplies the diode rectifiers 3|, 32 with unequal voltages and the average potential drop across the resistors 33, 34 consequently is no longer equal although the polarity of each potential drop does not change.

' The magnitude of the difference, and thereby the magnitude of the control potentia1 between the conductor 66 and ground, obviously varies in direct relation to the magnitude of the intermediate frequency departure. The automatic frequency control circuit conductor 66. now either positive or negative with respect to ground, depending upon the direction in the frequency spectrum of the local oscillator frequency departure, carries a control potential which adds, with proper polarity, to the biasing potential previously impressed on the grid 48 to bias this grid to a new operating potential. Assuming that the conductor 66 is now positive with respect to ground, the grid 48 is biased more positively than it previously was and the space current flowing from the anod 55 (and from the screen grid 53) to the cathode 56 of the device 49 thereupon increases to a new value. This increased space current flows through the resistor 63 to effect a change in its resistance and, the resistor '63 being connected in shunting relation to the transformer winding 6|, a consequent change in the reactance of the circuit 69 is reflected by the winding 6| into the oscillatory circuit l8, I9 of the local oscillator 2|. The local oscillator 2| thereupon changes its output frequency in a direction to offset the frequency drift thereby to restore the intermediate frequency to its former value.

It should be noted at this point that the conrtrol potential appearing between the conductor 66 and groundv is very greatly amplified, in its effective control over the resistance of the resistor 63, by Virtue of the amplifying characteristic of the electron discharge device 49. It will therefore be evident that, with this arrangement, the electron discharge device 49 operates with the dual function of amplifying audio frequency signal oscillations while at the same time amplifying the effective control of the automatic frequency control potential. Thus very small displacements of the intermediate frequency from a normal value result in large changes in the value of resistance of the resistor 63 which, when reflected into the oscillatory circuit of the local oscillator 2|, thereby effect substantial changes in the frequency of the local oscillator in a direction tending to restore the intermediate frequency to its former normal value.

The purpose of the resistor 35 is to off-balance to a certain extent the equality of the voltages produced across the resistors 33 and 34. This unbalance of voltages is only necessary when a single space current supply source is used for all of the tubes in the receiver and the regulation of the space current supply source is very poor. A poor regulation results in an increase in the space current of the electron discharge device 49 upon the tuning of the input circuits by the condensers 22 to receive a signal oscillation since the automatic volume control operates, when a signal is tuned in, to decrease the load on the space current supply source by decreasing the space current through the several electron discharge devices in the receiving apparatus l2 to which the automatic volume control circuit 12 is connected. Any increase in the space current of the device 49 in this manner is detrimental to the proper operation of my automatic frequency control and should be avoided. By using the resistor 35, a greater potential drop appears across the resistor 33 than across the resistor 34 when a signal oscillation is received and this excess voltage, since the ungrounded terminal of the resistor 33 is negative with respect to ground, causes the grid 48 of the device 49 thereupon to become biased slightly more negatively than formerly, even though the intermediate frequency is at its normal value. Changing the bias of the grid 48 in this manner effectively maintains a constant space current through the device 49 even though the potential of the screen grid 53 and the anode 55 rises by virtue of the poor regulation of the space current supply source. The same result may also be accomplished by eliminating the resistor 35 and by so unbalancing the center tap of the winding 26 of the intermediate frequency transformer 24 that a greater voltage is produced acrossthe resistor 33 than is produced across the resistor 34 at times when the intermediate frequency has a normal value,

It will be evident in the above arrangement that a certain amount of radio frequency energy will be induced into the automatic frequency control circuit 66 from the local oscillator 2|. This induced energy flows through the resistor 63 to produce a change in the value of resistance of the resistor 63 in much the same manner as the control current which flows through the conductor 64 from the anode and screen grid circuits of the electron discharge device 49. My arrangement has the important advantage that the induced current in the circuit 60 from the local oscillator 2| may be reduced to a small proportion of the total current which flows in the resistor 63. This is accomplished by giving the resistor 63 a relatively high value of resistance and by suitably reducing the magnetic coupling between the winding I8 and the winding 6| of the transformer |4. Since the inductive reac- Itance of the winding 6| is necessarily increased to match the high value of resistance of the resistor 63, the loosened magnetic coupling does not affect the total reactance reflected into the oscillator tuned circuit l8, l9 and therefore does not affect the relatively large range of frequency control which the control circuit 6!] has over the frequency'of the local oscillator 2|.

My automatic frequency control circuit has an additional advantage over the arrangement of said Goldman application previously referred to in that a lamp filament, which preferably constitutes the resistor 63, has a relatively large thermal time lag which is not possessed by the thyrite resistor of the Goldman arrangement. The thermal time lag of the lamp filament effectively prevents the audio frequency components of the control current appearing in the control circuit 64 from frequency modulating the local oscillator at an audio frequency rate. It is, therefore, possible to dispense in my arrangement with all but a relatively simple and inexpensive audio frequency filter which may be comprised by the resistor 65 and the condensers 5i and 62.

Fig. 2 shows a modification of my invention in which the amplification of the automatic frequency control potential is accomplished in an intermediate frequency stage of amplification. Elements in this figure corresponding to like elements of Fig. l are designated by like reference characters. The stage of intermediate frequency amplification, which would be part of the apparatus 12 of Fig. 1, includes an electron discharge device 13 which has a control grid 14 connected to a doubly tuned intermediate frequency transformer T5, and an anode 16 connected to a' frequency discriminating network, represented conventionally by the rectangle 83. The device 13 also has a cathode 11 with a suppressor grid 18 connected thereto and a screen grid 19 connected through a resistor 80 to the positive terminal 8| of a high voltage space current supply source. In this modification, the automatic frequency control potential is supplied through a control conductor 66, a filter comprised by the resistor 61 and the condenser 68, and through the secondary winding of the intermediate frequency transformer to the control grid '54 of the electron discharge device '13. The cathode circuit of the device 73 is completed from the cathode I! through a resistor 65 and a conductor 54 and through a frequency control arrangement of the same type as that designated by the numeral 60 of Fig. 1 but which is here conventionally represented as being included in the rectangle 82 which also includes the local heterodyning oscillater 2! shown in detail in Fig. 1. The device 13 thus amplies both the intermediate frequency oscillations supplied to its control grid 14 and also amplifies the automatic frequency control potential impressed on the same grid. The operation of the Fig. 2 arrangement is the same as that of Fig. 1 and will not be described in detail.

It will now be apparent that my invention provides a greatly simplified automatic frequency control arrangement and accomplishes the amplification of an automatic frequency potential in a simple manner requiring no circuit elements in the receiver other than those already present.

While I have shown by way of illustration my invention as utilizing either an audio frequency or an intermediate frequency amplifier tube to suitably amplify the automatic frequency control potential, it will be evident that such control potential may be amplified by the use of an amplifier tube positioned in the radio frequency stages of amplification or by the use of any tube whose connections and operation permit the tube simultaneously to amplify both alternating current and direct current potentials supplied to its input circuit. It will be further evident that the arrangement which I show for amplifying an automatic frequency control potential is equally useful for the purpose of amplifying an automatic volume control potential or the like.

Thus, while I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the arrangement and in the instrumentalities employed, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

I. The combination of a tunable circuit having a resonant frequency, a resistor whose value of resistance between two points changes with changes of temperature of said resistor, means responsive to the resistance of said resistor between said two points for controlling the resonant frequency of said tunable circuit, and means to impress a variable voltage between said two points to change the temperature of said resistor to tune said tunable circuit.

2. In combination, an oscillator, an incandescent lamp, means to control the frequency of said oscillator in response to variations in the resistance of the filament of said lamp, and means responsive to variations in the frequency of said oscillator to vary the resistance of said filament of said lamp.

3. In combination, an oscillator, a resistance having a value dependent upon the temperature thereof, means connected across the terminals of said resistance for controlling the frequency of said oscillator in response to changes in the value of said resistance between said terminals, and means responsive to changes in the frequency of said oscillator to vary the temperature of said resistance between said terminals.

4. In combination, an oscillator having a frequency determining reactance, an incandescent lamp having its filament connected across said reactance to control the frequency of said oscillator in accordance with the resistance thereof, means to supply current to said filament to produce heating thereof, the resistance of said fila- 

